Development of phase shifter and variable true time delay devices is of interest for beam steering of phase-array antennas at microwave frequencies. Phase shifter and variable true time delay technologies include, but are not limited to, those based on FE/PEs, laser diodes, MEMS, interferometry, pin diodes, and ferrites. The objects of these technologies are the reduction of cost, power consumption, weight, and size of devices, while maintaining or exceeding performance, frequency range and bandwidth of existing devices. Cost of device architecture and control circuitry, device bandwidth, material permittivity/permeability, loss factor, conductor losses, and impedance mismatch are all considerations that must be factored into the decision to choose materials and designs. However, all of the above technologies have disadvantages that must be overcome before affordable high performance broadband electronically scanned antenna systems may be realized.
In an effort to overcome the disadvantages of the above technologies, and to provide affordable high performance electronic beam steering of antennas at microwave frequencies, FE/PE materials derived from Ba1-xSrxTiO3 (BST) have been previously proposed and investigated. Such FE/PE phase shifters are based on the change in the effective electrical length of the device due to a change in the material permittivity induced by applying a dc electric field with field strength in the range 0.1 to 100 V/μm in the material. From a device design and operation point-of-view, physical parameters of interest are the real permittivity (hereinafter referred to as dielectric constant is ε) of the material, tunability of the material, and microwave loss of the material as functions of both temperature and frequency. The ability to impedance match to the device is improved if the permittivity (ε) of materials used is relatively low; however, high ε doesn't always dictate a decrease in phase shifter performance.
Large tunability is desirable for shortening the length of line needed to provide the identical phase shift for each fixed electric field strength in a phased-array antenna. Lower loss tangent materials can further reduce phase shifter losses in the RF, microwave and millimeter wave range if appropriate tunability can be maintained. Dielectric constant, tunability and loss tangent can all vary significantly with both frequency and temperature.
All three of the above parameters of a FE/PE material increase and peak as the Curie temperature, Tc, is approached. If the Curie transition is sharp, the material will have a limited temperature range for operation because the tunability rapidly approaches zero, as demonstrated in FIG. 2, for temperatures away from the Curie point. Below the Curie temperature, the FE/PE material again exhibits reduced tunability and losses can be unacceptably high. Thus, conventional FE/PE phase shifters are typically operated at T>Tc, where they are paraelectric (PE).
Therefore, it is an object of the present invention to provide a FE/PE material whose dielectric constant has a gradual peak, as shown in FIG. 2, at the Curie temperature, and is as low a dielectric constant as possible, in order to overcome the above deficiencies of previous FE/PE materials and to provide a more constant ε, broader temperature range of operation and improved impedance match for the device. Other objects of the present invention include providing higher change in dielectric constant for a given change in magnitude of dc electric field applied to the material and, for some applications, higher rather than lower dielectric constant.
The effects of structure on the FE/PE transition have been previously analyzed for a variety of cation substitutions into BaTiO3. It was found that covalency can influence lattice distortion, spontaneous polarization and Tc. In the previous work, Tc was correlated with chemical bonds. The various parameters considered were size, charge, coordination number, electronic configuration of the cations, bond covalency, and cationic ordering. It has been suggested in those previous studies that Tc may be a function of octahedral distortion; values might be adjustable with a minimal number of parameters compared to band structure analysis.
Several methods have been previously used for synthesizing FE/PE materials, such as producing perovskite oxide materials for use in microwave phase shifters. For example, composites comprising magnesia (MgO) and a solid solution of BST have been produced by making a simple weight percent addition of MgO to BST. However, temperature-dependent data of the low permittivity and tunability of the same indicate that such material may have limited use over the military specified temperature range of operation (−50° C. to 100° C.). In addition, two-phase composites may also complicate methods to effectively deposit thin films of the FE/PE material for devices, particularly if the MgO and perovskite are not compatible for simultaneous deposition or simultaneous adhesion with a substrate, or are not compatible with each other.
In addition to two-phase composites of MgO and perovskite, previous attempts have been made to dope perovskites with Al2O3 to increase tunability. Significant tunability increases have been observed. However, results were only obtained at room temperature using low doping on the order of 1% to obtain single-phase material, and it is believed that this material has a sharp Curie transition temperature and suffers from a limited temperature range of operation.
Compositions in which charge-compensated substitutions into the A and B site have been produced, referred to as acceptor-donor co-doped BaTiO3, the focus of the producers thereof attempting to achieve high tunability and low dielectric loss tangents over a broad temperature range that includes the mil spec range. However, methods to achieve reduction of ε or constant ε, and tunability versus temperature, were not investigated.
The effects upon Tc of substitutions of Zr and of Sn for Ti in BaTiO3 have been previously studied, and the development of new materials in which cation size and change affect how Tc is shifted have been considered, but tunability of such systems has not been previously analyzed. The effects of structure on the FE/PE transition have been analyzed for a variety of cation substitutions into BaTiO3, and it was found that covalency can influence lattice distortion, spontaneous polarization and Tc. Further, Tc was correlated with chemical bonds. The various parameters considered were size, charge, coordination number, electronic configuration of the cations, bond covalency, and cationic ordering, and it was suggested that Tc may be a function of octahedral distortion; values might be adjustable with a minimal number of parameters compared to band structure analysis.
While FE/PEs afford many advantages for electrical beam steering of antennas, one major disadvantage of current FE/PE phase shifters is the temperature sensitivity of phase shifter devices due to the sensitivity of permittivity (dielectric constant) to temperature variation. In order to control temperature variation, temperature-compensation circuits, ovens, cooling systems and the like are currently needed. Thus, it is an object of the present invention to provide a FE/PE material, for electric-field tunable devices such as phase shifter applications, having uniform permittivity and tunability throughout the temperature range of operation, i.e., a material that is relatively insensitive to temperature variation or equivalently having a small temperature coefficient of permittivity and tunability over the desired range of operating temperatures. Such a FE/PE material would simplify electric control circuitry so that temperature-compensation circuits or ovens can be eliminated, thus reducing overall cost and size of devices.